1. Field of the Invention
The present invention relates to condition-sensitive switching structures and methods and refers more specifically to a light-sensitive electrical switch and method useful for controlling street lamps or decorative and/or safety lighting so that they are energized during the hours of darkness.
2. Description of the Prior Art
The structures commonly used at the present time for controlling street lights include photo-sensitive elements. These structures are simple, inexpensive, and operate automatically and reliably for a number of years. They generally consist of a photo-sensitive element the resistance of which varies inversely as the light striking it, a time delay relay with normally closed contacts to control the load, and occasionally, some resistors to control the current through the operating coil of the relay. The time delay feature is needed to prevent the control from responding to transient changes in light level such as may be caused by lightning flashes or automobile headlights. The normally closed contacts provide "fail-on" operation of the street lights in response to the most likely failure modes of the control.
Adjustment of the turn-on light level in accordance with prior methods for controlling street lights is made by adjusting the drop-out current through the relay. This may be accomplished by controlling the light which strikes the photo-sensitive element, limiting the relay current with resistors, and/or mechanically adjusting the force required to operate the relay contacts. Once the turn-on light level has been selected, the turn-off light level is automatically fixed also. And, since the relay's pull-in current is always higher than the drop-out current, the turn-off light level will always be higher than the turn-on light level. This turn-off light level to turn-on light level ratio, typically, varies anywhere from 2:1 to 5:1.
For a number of reasons, it is desirable to turn on street lights in the evening earlier than when they are actually needed. These reasons have to do with the physiological effects that the waning evening light has on the human eye and also with the fact that some of the street lights, presently in use, require a period of warm-up time before they reach full brilliance. However, this means that the lights are turned off in the morning at a light level which is from two to five times higher than the turn-on level in the evening. In fact, it is satisfactory to turn the lights off in the morning at a light level which may be three or four times less than the turn-on light level in the evening. Thus, a light-sensitive switch with a turn-off to turn-on ratio of less than 1:1 would be desirable. Ratios of 1:3 or 1:4 would be ideal. Such a control would promptly turn off the lighting load when it was no longer needed, thereby saving substantial amounts of energy.
The prior art discloses efforts to address this problem by designing light-sensitive switches with off-on ratios as close to 1:1 as possible (see U.S. Pat. No. 3,351,762 and U.S. Pat. No. 4,095,100 for a description of some of these efforts). The difficulty with these controls is that their operation is likely to become unstable. When the light level is near the operating point in the evening or in the morning, slight variations in light intensities, such as may be caused by clouds, may cause the control to cycle on and off. This mode of operation is disturbing to the viewer and may also cause damage to some of the lights presently in use. No efforts to devise a control with an inverse turn-off to turn-on ratio (that is a control wherein the turn-off light level is lower than the turn-on light level) are disclosed in the known prior art.